1. Field
The present invention relates generally to fifth wheel assemblies, and in particular, to fifth wheel assemblies with removable bearing plates.
2. Description of the Related Art
Fifth wheel hitches for towing a trailer behind a towing vehicle have long been known in the art. Examples of fifth wheel hitches include those disclosed in U.S. Pat. Nos. 4,721,323, 5,449,191, and 5,707,070.
With reference to prior art FIGS. 1A-1E, large over-the-road freight trailers 119 are usually coupled to tractor trucks 109 by means of a fifth wheel coupling 101 whereby the truck rear drive axles directly support a portion of the trailer 119 load burden. Usually, the fifth wheel coupling 101 and support point is located along the length of the tractor 109 between the rear drive wheels and the front steering wheels thereby distributing the front trailer 119 load. It is desirable for the fifth wheel coupling 101 to be adjustable in the longitudinal position with respect to the truck 109 to aid in control over distribution of the load.
Typically, a fifth wheel coupling 101 includes a support frame 121 for mounting the coupling 101 to the towing vehicle 109, such as, for example, to a tractor truck. This support frame 121 includes a pair of base rails 102 that are usually bolted to the bed and/or frame of the truck and that are tied in parallel by tie bars 104. Side brackets 155 are mounted to the base rails, either directly or through a pedestal 117 which may be longitudinally adjustable. A fifth wheel 130 (FIG. 1C) is mounted to the side brackets 155 by means of a trunnion arrangement (not shown) allowing for pivotal fore-and-aft movement. The fifth wheel 130 comprises a top bearing surface 145, or plate, and a slot 135 opening toward the aft end of the towing vehicle for receiving a trailer kingpin, the slot bounded laterally by a pair of ramped tips 132a, b. Fifth wheel 130 is an assembly which includes a jaw assembly, or locking mechanism (not shown) that is specifically adapted to releasably engage and hold a kingpin 115 of a trailer desired to be towed by the towing vehicle 119.
Prior art fifth wheels are typically formed of steel or ductile iron. FIG. 1E depicts the underside of a typical prior art fifth wheel 130 which incorporates an integral load structure comprising a set of walls 126a-d that serve to transfer the loads experienced by the bearing plate to the support and then to the tractor, and to house the kingpin locking mechanism. The load structure further comprises pivot bearings 127a,b which rest upon the side brackets 155 and permit pivotal movement of the bearing plate 130 in the vertical plane. Heretofore, fifth wheels have been manufactured such that the bearing plate and load structure are integrated, either through casting to produce a unitary fifth wheel, or by welding the two components together.
If, however, a fifth wheel bearing plate is damaged or worn beyond acceptable limits, the entire fifth wheel must be replaced whether or not the load structure underneath is still operable. Also, these methods of manufacture require that the two components be of the same or similar materials. Since the load structure is most often steel, then the bearing plate must also be steel. This means that the fifth wheel is a weighty device.